Industrial gas turbines often are capable of alternatively running on liquid and gaseous fuels. These gas turbines have fuel supply systems for both liquid and gas fuels, e.g., natural gas. The gas turbines generally do not burn both gas and liquid fuels at the same time. Rather, when the gas turbine burns liquid fuel, the gas fuel supply is turned off. Similarly, when the gas turbine burns gaseous fuel, the liquid fuel supply is turned off.
Gas turbines that burn liquid fuel systems require a liquid fuel purge system to clear the fuel nozzles in the combustors of liquid fuel. Similarly, a water purge system is required to purge the water injection system that is often used to add water to the combustion chamber when a gas turbine runs on liquid fuel. The water injection and the liquid fuel supply systems are generally turned off, when a gas turbine operates on gaseous fuel. When these systems are turned off, the purge systems operate to flush out any remaining liquid fuel or water from the nozzles of the combustor and provide continuous cooling air flow to the nozzles.
FIG. 1, shows schematically a gas turbine 100 having liquid fuel system 102 and a liquid fuel purge system 104. The gas turbine is also capable of running on a gas, such as natural gas, and includes a gaseous fuel system 106. Other major component parts of the gas turbine include a main compressor 108, a combustor 110, a turbine 112, and a controller 114. The power output of the gas turbine is a rotating turbine shaft 116, which may be coupled to, for example, a generator that produces electric power.
In the exemplary industrial gas turbine shown, the combustor may be an annular array of combustion cans 118, each of which having a liquid fuel nozzle 120 and a gas fuel nozzle 122. The combustor may alternatively be an annular chamber. In the combustion can arrangement shown in FIG. 1, combustion is initiated within the combustion cans at a point slightly downstream of the nozzles. Air from the compressor 108 flows around and through the combustion cans to provide oxygen for combustion. Moreover, water injection nozzles 111 are arranged within the combustor 110 to add energy to the hot combustion gases and to cool the combustion cans 118.
FIG. 3 shows a conventional purge systems for liquid fuel and water injection systems. The liquid fuel system 102 has an associated liquid fuel purge system 104, and a water injection system 124 and a water purge system 126. When the gas turbine 100 operates on natural gas (or other gaseous fuel), the liquid fuel purge and water purge systems 104, 126, blow compressed air through the liquid fuel and water injection systems and liquid fuel nozzles 120 to purge liquid fuel and water from the liquid fuel system 102 and water injection systems 124, respectively, providing continuous cooling air flow to the nozzles.
The air used to purge the liquid fuel and water injection systems is supplied air from a dedicated motor (M) controlled purge compressor 128. The purge compressor boosts the compression of air received from the main compressor 108. A compressor air pre-cooler 164, separator 166, and a filter arrangement 168 are used to treat the compressor air before it is boosted by the purge compressor. The purge air from the purge compressor is routed through piping 130, through temporary strainer 162, then to a tee 137 that splits the purge air flow and routes the flows to both the liquid fuel purge system 104 and water injection purge system 126. The tuning orifice 132 is used to meter the flow of purge air to the water injection purge air manifold 136 and nozzles 111, and the liquid fuel purge multiport valve 138. The liquid fuel purge multiport valve routes boosted pressure purge air to each of the liquid fuel nozzles 120. In addition, the purge compressor also provides air through another tuning orifice 133 to the atomizing air manifold 134 and to the atomizing air ports of the liquid fuel nozzles 120.
When the liquid fuel purge system 104 is initiated, a solenoid controlled soft purge valve 140 is open simultaneously with the multi-port valve 138 by a common solenoid valve 139. The soft purge valve 140 opening rate is mechanically controlled by a metering valve in an actuation line (not shown). The soft purge valve opens over a relatively long duration of time to minimize load transients resulting from the burning of residual liquid fuel blown out into the combustor from the purge system piping 142 and the liquid fuel nozzles. The soft purge valve 140 is a low flow rate valve, to reduce the boosted pressure purge air flowing from the purge compressors. After the soft purge valve has been opened a predetermined period of time, a high flow purge valve 144 is opened to allow the boosted purge air to flow at the proper system pressure ratio. The high flow purge valve may be a two-way ball valve 144.
When the water injection purge system 126 is initiated, a solenoid controlled three-way ball valve 146 is opened. The opening rate of this valve 146 is mechanically controlled by a metering, valve in the actuation line (not shown). The water injection valve 146 is slowly opened to minimize the risk that the boosted purge air will not cause a high flow of water to quench the combustion flame as the residual water is blown out of the purge system piping 148 and out of the water injection nozzles 111 in the combustion cans. The end cover check valves 147 prevent the backflow of liquids into the purge manifold 136 and multi-port valve 138.
The above-described piping, valves, purge compressor and other components of the purge systems for the liquid fuel and water injection systems are complicated and cumbersome. They require controlled opening of several valves, multiport valves, metering tuning orifices, check valves, all of which require maintenance and are possible failure points. If these purge systems fail, component failures will likely go undetected until turbine operation is ultimately affected, at which time the turbine must be taken off-line and serviced. To avoid having to take a gas turbine off-line due to a purge system failure, the conventional wisdom has been to add more purge system components and to add a backup system to the main purge system.
For example, if the purge compressor 128 fails, then air for the purge systems is supplied from an atomizing air compressor 150. Air from the atomizing air compressor is cooled in a purge air cooler 152. When the atomizing air compressor operates to provide air for the purge systems, then motor (M) operated valves 154, 156, are closed to reduce flow and pressure, then air is routed through the purge cooler at the appropriate pressure and temperature.
In addition, motor operated valve 158 is opened to provide a surge protection feedback loop. The operation of these valves 154, 156 and 158 controls the air flow to and from the atomizing air compressor 150.
Purge air from the atomizing air or purge air compressor passes through a temporary strainer 162 to remove contaminants from the purge air and protect the contaminant sensitive components from start up and commissioning debris. The purge cooler 152 is in addition to the precooler 164, separator 166 and filter 168 used to cool air from the main compressor 108 discharge.
The previously-described conventional liquid fuel and water injection purge systems have long suffered from several disadvantages and are prone to failure. For example, they require a purge compressor to boost the compressed air from the main compressor. To overcome the disadvantages of prior systems, the conventional wisdom has been to continually redesign the components of these purge systems, especially those components, e.g., check valves 147 and multi-port valve 138, that are prone to failure due to contaminants in the purge air.
The conventional control method has been to utilize a series of tuning orifices to balance the purge air and to set the appropriate pressure ratios for acceptable combustion dynamics. These tuning orifices have had to be individually sized to adjust the pressure ratios of the purge air.
Furthermore, the conventional purge systems require subsystems, such as a soft purge valve 140 with tuned needle valves, for initial application of purge air to the nozzles of the liquid fuel system. The soft purge valve was added to minimize transient load spikes during fuel transfers when the purge systems are started. The water injection purge valve actuation system was also modified to include a three way ball valve 146, with tuned needle valve, to reduce the risk of a combustion flame out.
With the addition of purge compressors, backup systems for the purge compressors, tuning orifices, temporary strainers, subsystems and other new components, instrumentation had to be added to protect the new components against contamination. Also, a recirculation line 170 was added around the atomizing air compressor for surge protection. These fixes to the purge systems were marginally acceptable. The conventional purge air systems, with all of their fixes and new components, were complex, delicate and not adequately reliable.